1. Field of the Invention
The present invention relates to an extreme ultraviolet light source device, a laser light source device for the extreme ultraviolet light source device, and a method for adjusting the laser light source device for an extreme ultraviolet light source device.
2. Description of the Related Art
A semiconductor chip is created, for example, by reduction projection of a mask, on which a circuit pattern is drawn, onto a wafer coated with resist, and repeating such processing as etching and thin film deposition. As semiconductor processing increases the degree of miniaturization, light with an even shorter wavelength is demanded.
To meet this demand, a semiconductor exposure technology for using light with an extremely short wavelength, 13.5 nm, and a reduction optical system, is under consideration. This technology is called EUVL (Extreme Ultraviolet Lithography). Hereafter extreme ultraviolet light is referred to as “EUV light”.
As the EUV light source, an LPP (Laser Produced Plasma) light source, a DPP (Discharge Produced Plasma) light source and an SR (Synchrotron Radiation) light source are known.
The LPP light source is a light source which generates plasma by irradiating a laser beam onto a target material, and using the EUV light radiated from this plasma. The DPP light source is a laser source which uses plasma generated by discharge. And the SR light source is a light source using orbital radiation light. Among these three types of light sources, the LPP light source has a high possibility to provide high output EUV light, since the plasma density can be increased and a solid angle for collection can be increased more than the other types.
In order to obtain a high output driver laser beam at a high repetition rate, a laser light source device constituted based on an MOPA (Master Oscillator Power Amplifier) system has been proposed (Japanese Patent Application Laid-Open No. 2006-128157).
In the LPP light source, a saturable absorber can be used so that a laser beam, reflected by the target material in the chamber and returned to the optical path (so called “return light”), parasitic oscillation light and self-excited oscillation light in the amplifier are absorbed.
The saturable absorber has a characteristic to absorb a laser beam having an intensity less than a predetermined value. By using a saturable absorber, damage to an amplifier and laser oscillator can be prevented, and the quality of a laser beam can be increased by removing small pulses called “pedestals”. A technology on a saturable absorber is known, however which is not a prior art on an extreme ultraviolet light source (U.S. Pat. No. 3,638,137).
For an extreme ultraviolet light source device, a carbon dioxide laser (hereafter CO2 laser) is used at high output (pulse energy 100 to 200 mJ) at high repetition rate (10 to 200 kHz). An extreme ultraviolet light source device is demanded to have a capability to supply a laser beam having stable pulse energy and pulse waveforms for a long time.
In order to use a CO2 pulse laser stably for a long time in such a high load state (state where high repetition rate and high pulse energy are required), oscillation due to parasitic oscillation, self-excited oscillation and return light must be suppressed. A possible method is to dispose a saturable absorber on the optical path of the laser beam in order to prevent the parasitic oscillation and self-excited oscillation so that the pulse energy and pulse waveform are stabilized.
In the prior art (U.S. Pat. No. 3,638,137), however, an object of the art is not using a saturable absorber in order to stably use a high output CO2 pulse laser for a long time. Therefore in this prior art, a gas cell is disposed in a resonator of the CO2 laser, and a cylinder of SF6 (sulfur hexafluoride) and a cylinder of C2F3 are connected to the gas cell, so as to supply a mixed gas (saturable absorber gas) of SF6 and C2F3 to the gas cell. The mixed gas supplied to the gas cell absorbs a laser beam less than a predetermined value emits heat, and then is exhausted from the gas cell.
In the prior art, parasitic oscillation, self-excited oscillation or oscillation due to return light can be prevented by supplying mixed gas to the gas cell until gas in each cylinder is emptied. However the amount of gas that can be stored in a gas cylinder is limited, and the mixed gas that is once used is exhausted, so gas in each gas cylinder will eventually be used up. Once each gas cylinder is emptied and can no longer supply mixed gas to the gas cell, the gas cell cannot normally perform the expected function.
It is also possible to seal the mixed gas in the gas cell, or to decrease the flow rate of the mixed gas supplied to the gas cell. In these cases, however, the temperature of the mixed gas in the gas cell rises when the laser beam passes through, and the mixed gas decomposes and can no longer play the role of a saturable absorber. In this way, the problem of the prior art is that the saturable absorber gas cell cannot be operated stably for a long time.